Re: 27 VAC to 32 VAC?

"James Thompson" <Jamesthompson2002@xxxxxxxxxxx> wrote in message
news:45ec9$44406307$438c8615$12873@xxxxxxxxxxxxx

"Chris" <cfoley1064@xxxxxxxxx> wrote in message
news:1145067561.776671.208370@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Dan Beck wrote:
Hello all,

I am a lurker novice with a few questions. I have a transformer
(specifically out of a pinball machine) that outputs 27 VAC, and it is
fused
for 8 amps in the circuits it supplies. I would like to change the
output
to 32 VAC, for a circuit fused at 0.25 amps. Is this technically
feasible,
or is my novice understanding currently embarrassing me (without me
knowing
:-)? I have "cut and pasted" the material below from a previous post.
Is
this technology relevant to my question? If so, would I incorporate
this
chip into a homemade circuit board with other devices to plug in line to
the
27 VAC output? I am capable of the tech/soldering/schematic reading,
but I
am way short on the engineering.

National Semiconductor have a handy online tool to help you use their
ICs<<

http://www.national.com/appinfo/power/<<

Thank you in advance for reading, and for any thoughts you may have.

Regards,
Dan

Hi, Dan. The National Semiconductor ICs are for converting a DC
voltage to another DC voltage. You've got AC, so your problem is
different.

If your transformer is rated for 27VAC at an 8 amp load, the voltage
will be quite a bit higher if it's unloaded or lightly loaded (1/4A is
just 3% of the transformer rated load). You may already be close
enough -- you should try it and see.

If it's not high enough, though, you'll have to do something else. The
easiest way to vary the secondary voltage in a transformer is to vary
the primary voltage. That would mean you have to bump the primary
voltage up to 119% of nominal (about 142VAC for 120VAC nominal
primary). That's too high -- the transformer isn't meant to work at
that high of a primary voltage. However, as noted above, the secondary
will almost certainly be high for your light load, and you might be
able to get away with 10% overvoltage or less. That would be
acceptable. You can do this with an autotransformer.

If this isn't acceptable, you can add the secondary voltages of two
transformer secondaries. You'd need to find a second transformer with
a secondary voltage around 5VAC, and connect the two secondaries
together to get your 32VAC, like this (view in fixed font or M$
Notepad):

|
| L1 _ 27VAC _
| o----o---o_/ \o--. ,---------o_/ \o-----o
| | FU1 )|( FU3
| L2 | )|(
| o-o--)-----------' '--.
| | | T1 | 32VAC
| | | _ |
| | '---o_/ \o--. ,--'
| | FU2 )|(
| | )|(
| '--------------' '--------------------o
| T2
| 5VAC
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Be sure to measure the output voltage. If your output voltage is more
like 22VAC than 32VAC, it's bucking instead of boosting -- reverse the
leads of the second transformer.

I hope this has been of help. Feel free to post again if you have any
other questions.

Good luck
Chris

I agree with chris in that your needs as far as what you are running, can
still operate on the 27 vac. What is the circuit you intend to run that
is needing 32vac?


Hello James,

thank you for your attention to my queries. I am wishing to build a test
device for pinball machine circuit boards, for a particular series of
electronic pinballs. Within this series there were a number of iterations
of score displays, used in the games. The transformer I have does not
provide the necessary voltage for the newer type of score display I would
wish to test. The new type of display in question requires 32 VAC; this
voltage is subsequently (on the score display pcb) rectified and
"boosted/bucked" (I hope I have those terms correct) to -45 VDC and -15 VDC
respectively. These voltages are then used by the digit driver ICs on the
pcb. I am sorry for the long-winded answer; I felt some background was
necessary. You may be correct, the 27 VAC may be enough; I just haven't
built the thing yet to test. What do you think? I will also add more
material in response to Chris's post above.

Regards,
Dan


.

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